Summary of Senior Notes Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Debt Outstanding [Line Items]
Senior Notes	$ 1,497.0	[1],[2]	$ 1,996.6	[1],[2]
Fixed Rate Corporate Senior Notes 3.45 Percent Due November 2020
Debt Outstanding [Line Items]
Interest Rate	3.45%	[1],[2],[3]
Senior Notes	499.6	[1],[2],[3]	499.5	[1],[2],[3]
Fixed Rate Corporate Senior Notes 3.38 Percent Due August 2021
Debt Outstanding [Line Items]
Interest Rate	3.38%	[1],[2],[4]
Senior Notes	498.5	[1],[2],[4]	498.3	[1],[2],[4]
Fixed Rate Corporate Senior Notes 2.38 Percent Due August 2022
Debt Outstanding [Line Items]
Interest Rate	2.38%	[1],[2],[5]
Senior Notes	498.9	[1],[2],[5]	498.8	[1],[2],[5]
Fixed Rate Corporate Senior Notes 4.63 Percent Due May 2014
Debt Outstanding [Line Items]
Interest Rate	4.63%	[1],[2]
Senior Notes			$ 500.0	[1],[2]

[1] Not redeemable prior to maturity.

[2] Debt issue costs are recorded as an asset and amortized on a straight-line basis over the life of the Note.

[3] Notes issued at a discount of 0.117%.

[4] Notes issued at a discount of 0.437%

[5] Notes issued at a discount of 0.283%